This invention relates to a photo-mask and in particular to a photo-mask to be used for the production of a substrate for an optical memory element.
Recently, the importance of optical memory elements as large-capacity high-density memory devices has been increasing. Depending on the manner in which they are used, optical memory elements can be divided into the following three categories: the read-only type, the add-on type and the erasable type. Of these, optical memory elements used as add-on or erasable memories are generally provided with guide tracks for guiding a light beam to a desired position thereon for recording, reading or erasing information as well as addresses for identifying the individual tracks. If tracks are further divided into sectors for managing the stored information, sector numbers may also be provided on the tracks.
One of the methods of producing such guide tracks (grooves) has been described, for example, in U.S. Pat. No. 4,544,443 issued Oct. 1, 1985 and assigned to the present assignee. Accordng to this method, a resist film 5 is applied to a glass substrate 4 as shown in FIG. 5(a) by using a spinner or the like and then it is exposed to a beam 8, for example, of ultraviolet light through a photo-mask 6. Guide tracks, guide numbers and sector numbers are preliminarily patterned on the photo-mask 6 such that they are transferred to the resist film 5. With reference to FIG. 5(b), parts indicated by the numeral 7 are partially removed light non-transmissive film of Cr, Ta or the like, forming a desired pattern to be transferred. After the resist film is developed as shown in FIG. 5(c), the pattern is directly etched on the glass substrate 4 as shown in FIG. 5(d) by a reactive ion etching method wherein an etching gas such as CF.sub.4 and CHF.sub.3 or by a wet etching method, for example, with a HF solution. Finally, the residual resist film is removed as shown in FIG. 5(e) by ashing in a O.sub.2 plasma or washing with a solvent such as acetone.
The photo-mask 6 used in the step corresponding to FIG. 5(b) can be manufactured, as shown in FIG. 6, by forming a resist film 11 on a light non-transmissive film 10 over a circular disk-shaped photo-mask substrate 9 and using a beam 13, for example, from an Ar laser with a converging objective lens 12 to produce a spiral or circular guide track on the resist film 11 while rotating the disc around its central axis.
FIG. 7 shows examples of cross-sectional structures of various photo-masks which have been used in semiconductor industries. FIG. 7(a) is an example wherein a single-layer film of Cr 10 is formed on a mask substrate 9. FIG. 7(b) is another example which shows a double-layer structure with Cr 14 and CrO.sub.x 15, the latter 15 being used as an antireflective layer. If reflectivity is too high when a master mask is manufactured with a photo-repeater, the image resolution of the manufactured pattern may sometimes become worse. When a pattern is transferred to a resist film on a silicon wafer, the pattern resolution may also become worse by the multiple reflections between Cr and the wafer. The antireflective film is intended to prevent such results in these applications. FIG. 7(c) is still another example wherein an InO.sub.3 film 16 is inserted between the Cr film 14 and the glass substrate 9.
If the method shown in FIG. 6 is used to form guide tracks or to record track numbers and sector addresses on a mask of the kind shown in FIG. 7, the Ar laser light used for this purpose becomes absorbed by the Cr film. This is particularly the case when an antireflective film is present as shown in FIG. 7(b). This causes an increase in the temperature of the resist film and the resist film becomes destroyed by heat. Edge surfaces of the recorded resist will become uneven as shown at 17 in FIG. 8. If a mask with resist film having such rough edge surfaces is used for the manufacture of optical memory elements as shown in FIG. 5, large noise will be generated in the signals recorded in the element because of the scattering of light by such uneven surfaces.